1. Field of the Invention
The present invention relates to a sampling frequency conversion apparatus for converting, e.g., a digital audio signal into a digital audio signal having a different sampling frequency.
2. Description of the Related Art
Conventionally, when a digitally recorded signal is converted into a signal having a different sampling frequency, the digital signal is temporarily D/A (digital-to-analog)-converted into an analog signal, and the analog signal is A/D (analog-to-digital)-converted at a different sampling frequency again. Although this method allows reliable conversion, the signal is deteriorated since D/A conversion and A/D conversion are performed. In addition, since an A/D converter and a D/A converter must be arranged, the signal tends to be deteriorated by mixing of, e.g., hum noise.
In consideration of the above situation, a D/D (digital-to-digital) conversion method for directly converting a digital signal to a digital signal having another sampling frequency has been put into practical applications.
In a general sampling frequency conversion apparatus employing the D/D conversion method, sampling pulses having a frequency corresponding to the least common multiple of input and output sampling pulses are generated. An input data string is interpolated at timings of the generated pulses, and data coinciding with the timings of the output sampling pulses are extracted and output from the interpolated data string. This method is effective when the input and output sampling pulses have a relatively large frequency difference, and the least common multiple can be easily calculated. However, when the frequency difference is small, the least common multiple becomes too large to realize the method.
In the conventional method, when input and output sampling pulses have a small frequency difference, an interpolated data string is generated on the basis of sampling pulses having a given frequency, and data closest to timings of the output sampling pulses are extracted and output. However, when such approximation processing is performed for, e.g., a digital audio signal, interference components which depend on the sampling frequency difference appear in an audio frequency band, resulting in deterioration beyond an allowable level in the auditory sense.